The present invention relates to a harmonic drive and an internal geared wheel for such a drive according to the respective preambles of the main claims.
Such a harmonic drive having an internal geared wheel, comprising a link chain made of toothed segments which enclose an springy steel sleeve, in turn supported via bearing rollers on a cam, which is put in rotation via a planetary gear arrangement situated inside it, driven using a central shaft, is known from German Patent Specification 1 182 011.
The functionality of such a harmonic drivexe2x80x94also known as a wave drive or ring band drivexe2x80x94as a very strongly reducing, self-limiting system having a driven shaft coaxial to the drive shaft, is based on a rotating wave generator (namely the cam in this case) continuously radially deforming an internal geared wheel hoop (in this case the chain, whose chain links are toothed on their outer curves) and therefore the external peripheral surface of this wheel rim continually pressing locally outward against the hollow cylindrical internal peripheral surface of slightly greater circumference of a stationary, dimensionally stable bearing ring. As a consequence, the internal geared wheel itself or its wheel hoop, mounted thereon as a rotatable wheel rim, rolls in the bearing ring frictionally via friction surfaces or (as in this case) positively via teeth, the wheel and/or its hoop rotating slower than the drive core of the wave generator, which is driven by a motor, according to the difference in circumferences.
This rotational movement, which is greatly slowed in relation to the actuation, is preferably transmitted via the external teeth of the hoop, which is shaped like a wheel rim, to the internal teeth of a further external ring, the driven ring, which is concentric to the bearing ring but not stationary, a further reduction of the rotational velocity able to occur due to differing circumferences (number of teeth). The wave generator is typically driven via a low voltage DC motor, coaxially flanged on, which is high-speed and therefore available very cheaply, whose rotation is thus reduced into a much greater torque corresponding to a much slower rotational movement.
The refinement of such harmonic drives according to German Patent 1 98 33 290 C1=European Patent Application 0 974 773 A concerns damping of the noise generation as a consequence of the revolving displacement of discrete tappets (instead of the rotating deformation of the closed hub of a spoked wheel) on the non-round wave generator. Since the feet of the individualxe2x80x94not guided in a cage, but rather connected to one another by a flexible ring bandxe2x80x94tappets are no longer solid, but rather split, they are able to elastically bend so they may press continuously against the drive core of the wave generator. The peripheral connection of the neighboring tappets via the ring band, which is thin in comparison to their diameters, is to be laid out in such a way that a spring effect is achieved, using which the tappetxe2x80x94even in the course of instantaneous reduction in diameter as the drive core, which is oval in cross-section, rotates out of the angular region of its greatest diameterxe2x80x94is always kept radially pressing snugly against the non-round wave generator in order to, in this way, suppress a noise-generating radial rebound from a displacement position which is still radially lifted.
In the embodiments known from German Utility Model 2 96 14 738 and described in more detail in the article xe2x80x9cGenial einfachxe2x80x9d by H. Hirn (KEM Antriebstechnik issue November/1996), the harmonic drive does not have a chain and planetary gear construction, which is noisy, subject to wear, and expensive because of the many individual parts to be assembled, but rather comprises very few plastic injection molded parts. As a wave generator, a non-round (for example triangular or preferably oval in axial cross-section) drive core is rotated concentrically in the hub of a radially deformable internal geared wheel. Dimensionally stable spokes between the hub, which is radially deformed by the drive core, and the hoop of this internal geared wheel, which is also radially deformable and is externally toothed, have the effect that the outer teeth of the hoop, corresponding to its revolving radial deformation, only engage over a limited curved segment, which rolls therein, with the inner teeth of the bearing ring. To reduce the heat-generating friction between the pairing of the plastic lateral surfaces, of the drive core on the one hand and of the internal geared wheel hub on the other hand, rotating relative to one another, a friction bearing in the form of a metal collar may be introduced, which also eccentrically deforms revolvingly under the influence of the eccentric drive core rotated therein (i.e., it does not have any support function for toothed chain links resting thereon according to the drive forming the species). This friction bearing, which is interposed in the plastic construction, distributes the relative rotation between the drive core and the internal geared wheel hub onto two stages and, merely due to the frictional behavior of plastic, which is generally more favorable than that of metal, also leads to reduction of the friction losses, i.e., the waste heat within this harmonic drive, which is compactly produced almost completely from plastic injection molded parts and is hermetically encapsulated. However, high demands are placed on the alternating stressability of the collar, which continuously deforms under load, and its installation in such a harmonic drive of small dimensions leads to a detectable elevation of the manufacturing costs. In addition, due to the high reduction ratio of such a harmonic drive, a heat buildup which is critical to its functioning may still occur, above all in the driving region.
In these known harmonic drives, which are compactly produced in plastic injection molding and to which high-speed and therefore cheaply available low voltage DC motors are typically connected in series, the internal geared wheel is therefore a one-piece injection molded part having dimensionally stable, spoke-like, radially oriented tappets between a radially deformable hub and the externally toothed hoop, also radially deformable, which requires a technologically demanding injection molding technique due to the material properties, which differ greatly in radial section. In this case, it has been shown to be economically disadvantageous, since it is expensive, ifxe2x80x94for example to adjust to concretely occurring operating requirements by replacing internal geared wheels of different circumferencesxe2x80x94differently dimensioned variants must be kept available.
The present invention is therefore based on the technical problem of maintaining the mechanical and application-oriented advantages of the previously known harmonic drives while refining them in such a way that their construction is simplified, in order to, above all, be able to be more flexible in regard to the implementation of the internal geared wheel through a more favorable implementation while reducing the production costs.
This object is achieved according to the present invention in that the harmonic drive according to the species and/or its internal geared wheel is designed using the features of the respective main claim. Accordingly, the idea of the toothed segments, from the chain construction forming the species, which has not yet received any practical significance, is inventively combined with the idea of the spokes from the injection molded implementation, which is also known, but has a very different construction, and which has been used in mechanical practice, in such a way that the internal geared wheel is assembled from a variable number of separately produced tappets having identical geometry, which are supported with the foot end of their shaft almost radially on the external lateral surface of the drive core and have opposing radially elastically compressible heads, only those of which lie in the extension of one of the larger radii of the wave generator, and which are therefore radially displaced somewhat outward from an imaginary circular segment, pressing snugly against the hollow cylindrical internal lateral surface of the external rings, which neighbor one another axially, (bearing ring and driven ring). The external teeth of the internal geared wheel may be implemented directly on the external lateral surface of the shaft heads, which neighbor one another at a distance; or the shaft heads are used as a support of a separate flexible band, which extends around the circumference over them, having external teeth. In the latter case, the flexible band may roll via the internal geared wheel, the internal geared wheel itself then does not need to rotate around the drive axle as well.
Approximately centrally and transversely, the shaft of each tappet has two short arms which end in connection profiles for articulated coupling to the peripherally neighboring arms of the neighboring tappets on both sides. Therefore, each of the tappets from which the internal geared wheel is assembled has the shape of a Western Roman cross in cross-section in relation to the drive axis, with a triangular expansion to the tappet head implemented above the vertical bar.
The internal geared wheel of a harmonic drive, which is subject to high mechanical stress, is therefore, according to the present invention, assembled in a pivotable and articulated way from individual radial tappets, which are identical to one another and essentially solid, each of which is equipped with an elastically radially compressible hollow head, using side connection profilesxe2x80x94preferably differing but completing one another complementarilyxe2x80x94which are connectable to one another in pairs. The geometry of the tappets may be produced economically on a large scale by plastic injection molding a rigid, thermoplastic, low-density material, in a technologically non-critical manner and with a considerable degree of dimensional accuracy, so that an internal geared wheel which may be assembled from a variable number of tappets is also distinguished by low mass inertia. Each tappet, which may be mounted in its longitudinal direction, has good sliding friction and wear characteristics and is very dimensionally stable in relation to heat, particularly in the foot region, when the foot is approximately radially supported on the wave generator, due to the combination of materials.
Reference is made, in regard to further characteristics and additional refinements of the present invention, to the following description of a preferred exemplary embodiment of the achievement of the object according to the present invention and to the further claims as well as to both of our contemporary parallel applications having the same title, particularly in regard to the wave generator support.